An antenna used in many radar systems, radio or communication devices is an electrically conductive device made of one or more electrically conductive materials and interfaces with a circuit and a medium surrounding the antenna, such as air or other dielectric medium, to either transmit an electromagnetic wave from the circuit into the medium or to receive an electromagnetic wave from the medium into the circuit. In transmitting the electromagnetic wave from the circuit into the medium, the circuit operates to generate an alternating current distribution at one or more alternating radio frequencies in the antenna which in turn radiates an electromagnetic wave at the one or more radio frequencies into the medium. In receiving an electromagnetic wave from the medium into the circuit, the antenna interfaces with the incoming electromagnetic wave at one or more radio frequencies to produce an alternating current distribution at one or more alternating radio frequencies in the antenna which is received by the circuit. In both transmitting and receiving operations, the antenna operates as a conversion device that performs conversion between the electromagnetic wave and the alternating current distribution.
Many antennas are made of electrically conductive materials such as metals. Electrically conductive materials are materials with high electrical permittivity such that the imaginary part of the electrical permittivity ∈″ is much greater than the real part of the electrical permittivity ∈′ (i.e., ∈″>>∈′). Magnetically conductive materials can also be used to construct antennas. Magnetically conductive materials are materials with high magnetic permeability that can be magnetized or de-magnetized under a magnetic field and tend to have the imaginary part of the magnetic permeability μ″ much greater than the real part of the magnetic permeability μ′ (i.e., μ″>>μ′), e.g., alloys of Fe, Ni and Co, or nickel zinc ferrite above the ferromagnetic resonance frequency.